Immunological factors seem to be important in determining the course and outcome of both acute and chronic viral hepatitis. Furthermore, promising therapies for chronic viral hepatitis have profound effects on immune function and sustained responses to therapy may depend largely on restoration of normal immune responsiveness. The role of immunologic mechanisms in determining the course and ultimate outcome of viral hepatitis is being studied and the effects of therapies on the immune system are being evaluated. Serial studies of cellular immune function were performed on patients with acute viral hepatitis and compared to analogous results obtained in patients with chronic type B hepatitis. In addition, the immunological status of patients with chronic type B hepatitis has been assessed and the effect of immunosuppressive as well as antiviral therapy on immunological function in these patients has been studied prospectively.